breather cap: why?

[falcon fanatic]

So, my valve cover is equipped with a PCV valve at the rear, and a vented cap at the front. Why would I want a vented cap? Doesn't the PCV valve provide all the venting my crankcase needs? And, doesn't the PCV valve assure vacuum in the crankcase by providing "positive crankcase" venting, and doesn't this providing better ring sealing? And doesn't having a vented cap negate the effects of having a PCV valve? Can I lose the vented cap, then? As far as I can tell, all it does is blow hot oily air into my air cleaner, which I can do without.

While I'm on the topic, my PCV does blow a bit of oil which naturally goes back into my intake manifold and throws off my mixture, so should I use an oil separator inline to mitigate the effects of that?

Finally, does it matter what PCV valve I use? the stock one is straight, so I went to Autozone and got one with a 90* bend. It fits better, but does it provide the right spring strength?

 

[fordconvert]

The PCV valve is just a suck thing so the air has to get into the engine somewhere. The earlier stuff used a combo filter cap. Later stuff used some sort of filter inline to the regular air cleaner. Most current stuff sucks the air into the crankcase from after the regular air cleaner. If you are getting oil out of the breather/ cap that means there is more blow by (from rings and such) getting into the crankcase than the system can consume under certain conditions.

The spring part of the valve is more or less a check valve that is intended to keep a carb backfire from getting into the crankcase. The differences (other than physical config and external size) is the internal passage size. From the factory the carb is tuned expecting that amount of what amounts to a vacuum leak. Going to a smaller valve could make you run on the rich side and a larger valve will suck more stuff through the crankcase but tend to push things leaner.

 

[frogmn666]

Don't know about your other questions, but I went with the 90° PCV valve too. Autozone had about 6 or more valves that all looked the same and were the same size on each end. No problem adjusting the idle mixture. I would like to know what the differences are in the PCV valves.

I found that the brass fitting at the carb adapter was fully plugged up with carbon and I was getting blow-by out the breather. Not anymore.

 

[Harte3]

I don't recall where I saw it but some were making a fluid seperator out of nothing more than a fuit jar and plumbing it in line between the PVC and the intake and some were using a seperator commonly found on air compressors to seperate oil and water from the air supply before it reached the tool being operated.

 

[frogmn666]

All you need is a container with an in and out port. The oil will fall to the bottom of the container. I would put a piece of foam rubber at the bottom to soak it up.

 

[66 Fastback]

This is a couple of posts I copied from the NCRS Corvette forum. The fellow positing this is an automotive engineer. I have a longer posts going into more details if interested.
Doug

I ran across a possible similar situation. Someone had installed a "correct reproduction" valve and the car did not idle correctly. I have a suspicion that someone is selling valves that are stamped with a "correct number", but they do not have the same flow rate as original.
I have seen the same issue with vacuum advance controls. The bracket was stamped with a correct OE GM number, but the VAC performance was nowhere near original.
The PCV valve is designed with a certain flow-manifold vacuum specification, and the carburetor is calibrated to this flow rate. A significant deviation from the correct PCV flow characteristics may not be compatible with the carburetor's idle adjustment speed and mixture range, which means the engine may not idle properly.
The most important priority is to have a correctly performing valve, so beware of "reproduction parts".
A PCV system yields less blowby dilution of the oil and less tendency for the engine to sludge. It also reduces HC release to the atmosphere and reduces fuel consumption since blowby (which consists mainly of unburned air-fuel mixture) is recycled into the inlet manifold for combustion.
There are no disadvantages for a properly designed PCV system although a poor design can ingest engine oil into the inlet system or not provide sufficient crankcase ventilation.

Your PCV system should have a fresh air intake from the air cleaner housing into the opposite valve cover.
The PCV system is designed to use manifold vacuum to continuously draw fresh filtered air into the crankcase, purging it of the combustible blowby gas, and then into the inlet manifold to be consumed by combustion.
PCV systems reduce HC emissions, help keep the crankcase free of harmful compounds that can cause corrosion and oil sludging, and improve fuel economy by consuming combustible blowby gas. There is no downside to them as with some other early emission control systems, and a properly functioning crankcase ventilation system is a must for both performance and engine longevity.

 

[60s Refugee]

If your PCV is stuck or wrong, it will cause a vacuum leak and cause a negative effect. If the PCV functions correctly, then you need a breather to allow fresh air to be sucked in as the PCV allows it to be sucked out. Of course, this means that the valve cover is where the area of the most air flow would be. We must presume that all that crappy stuff being sucked out must rise to the top of the engine all by itself.

I have seen many new and old 200's (with PCV systems) that have the residue coming from the breather and making a gradual mess of things. Most of the old restorable 200's will have a permanent oil stain at the front of the valve cover and around the filler cap area. This suggests a whole service life of oil mist escaping. This seems to be normal since they have more or less all done it, but it makes me wonder if the PCV really works at all, since it should suck stuff through the breather and into the engine and create some vacuum inside the engine, which you would think would keep stuff from flowing back out the breather. Engines were messy back in the day and we tend to forget that, especially since most folks don't drive their classics every day like I do.

I am about to install an aluminumumum valve cover and it lacks the baffle below the PCV hole that prevents oil from being splashed onto the PCV and sucked directly into the manifold from the rocker arm/valve stem lube action and such. Because of that I expect that using a PCV there will be detrimental. I plan to stick a KN or another type of breather in both holes. I'd like to keep the oil INSIDE the engine too, but I expect it will be the 'wipe it off the valve cover and stuff with a rag deal every oil level check' as is usually the case. I suppose that the air flowing through two breathers should at least vent some of it out, although into the atmosphere. I don't care so much about that cuz it will be blowing back on that Chevy guy who's trying to keep up with me!

If you delete your PCV thing, be sure to cap over the nipple on the carb spacer that the hose attached to in order to prevent a massive vacuum leak. I expect you already knew that, but ya' never know. Regardless, good luck and let us know how it works out!

Harry

 

[pachecoj]

I just want to preface this post by saying that I am not actually recommending this thing, just contributing some information...

Anyway, when I bought the car it had a wierd looking oil/water separator that you can see in this image:
http://s254.photobucket.com/albums/hh100/cbarnes69/?action=view&current=P1012551.jpg

It actually does pull a lot of stuff out of the line. I probably get about a cup of liquid in there (mostly oil, some water) after running for a few weeks. It's called the condensator:
http://www.condensator.com/

I was going to take it off because it looked silly, but after I noticed how much stuff it's pulling out I thought twice about it and left it on. I might replace it with something less conspicuous but for now it's still on there.

Jason

 

[frogmn666]

My `66 Stang with 128K had visible blow-by due to the plugged up brass fitting on the adapter. As I said, I went with the 90° valve and no longer have blow-by fron the filler cap. I get 19" of vacuum and the idle mixture screw is about at the same position.

As far as running rich or lean, I wonder if it matters at higher rpm.

On a GM LS1 engine, people complained about oil in the manifold from the PCV system. The cure was a PCV valve that only had a 3/16" hole in the big end. No guts, just a shell with the hole.

As far as the http://www.condensator.com/ Snake oil and overkill.

 

[Eric Rose]

That idea has been around for years, just a can with a hose running in and a hose running out. It's called a "catch can" and you can find them in any speed shop.

 

[falcon fanatic]

There seems to be a consensus that the reason the breather cap is there is to let air in so that the PCV can suck it back out. Sorry for saying so, but that makes no sense. Why would I want to bring air in just to let it out again? There seems to be plenty of "air" reaching the crankcase ust from blowby. Isn't the purpose of PCV to vent *this* air, and not just any air? By extension, then, the vented breather cap actually interferes with the function of the PCV.

 

[LaGrasta]

Wow, that is a lot of information for your original question. The short answer is "no".
My engine sucks in the air through the road draft tube (which is now just a crankcase filter and blows it out of the PCV (straight, angled or whatever shape fits) into the intake.

 

[LaGrasta]

I should also mention when my pcv had a bit of blowby around it, I was suspicious as it normally did not have it. I replaced the PCV and it's good again.

 

[60s Refugee]

Breathers were around a long time before PCV's were. They were originally, and still are, the most efficient way to vent off the blow by pressure and also provide a hole to pour oil into the engine. Trouble is, PCV's have check valves in them that close when the vacuum reaches a certain point, there by leaving the only way to vent the gasses as being through the breather. So at best, a PCV can only draw gasses into the intake while the vacuum is below the negative pressure that will shut the valve. That will mostly be at lower engine loads.

Again, I don't see where there is much benefit from having a PCV, but much to be lost performance wise if it is malfunctioning! IMO these were little more than a way to apease the tree huggers until a better system could be engineered. Our American engines went from being "lean and mean" to burning unleaded fuel and covered up with rubber spaghetti in only five years. It cost us our edge in the domestic autobusiness and ended the "horsepower" race practically overnight. It was a sad time for American quality control too. In the end, overall automobile quality worldwide drastically improved along with emmissions and gas mileage, but the whole bigger picture caught US by suprise. We were asleep, but we arn't anymore. American iron is made of aluminum and plastics now, but our industry is fighting back. Looks like Ford is leading the pack!

Still... I'd like to take my PCV and put it in a railroad track, then smile as a train flattens it into a more primal condition. In fact, I think I will! Oh well, hey, that's just me!

Harry

 

[falcon fanatic]

Wow Harry, that's a lot of vitriol for just a check valve and length of rubber hose

LaGrasta, when you say "no", which part of my long complicated question are you answering? Oh, and what's a "road draft tube"?

 

[pedal2themetal45]

Wow Harry, that's a lot of vitriol for just a check valve and length of rubber hose

LaGrasta, when you say "no", which part of my long complicated question are you answering? Oh, and what's a "road draft tube"?

Rosd Draft Tube
before PCV valve's the old I 6's (MY 61 170 had it ) there was just a metal tube that came out of the side of the block and went down beside the motor to just below it.. so the natural vacum from the low prusher under the car would pull the gasses out..
tim

 

[Bort62]

Breathers were around a long time before PCV's were. They were originally, and still are, the most efficient way to vent off the blow by pressure and also provide a hole to pour oil into the engine. Trouble is, PCV's have check valves in them that close when the vacuum reaches a certain point, there by leaving the only way to vent the gasses as being through the breather. So at best, a PCV can only draw gasses into the intake while the vacuum is below the negative pressure that will shut the valve. That will mostly be at lower engine loads.

Again, I don't see where there is much benefit from having a PCV, but much to be lost performance wise if it is malfunctioning! IMO these were little more than a way to apease the tree huggers until a better system could be engineered. Our American engines went from being "lean and mean" to burning unleaded fuel and covered up with rubber spaghetti in only five years. It cost us our edge in the domestic autobusiness and ended the "horsepower" race practically overnight. It was a sad time for American quality control too. In the end, overall automobile quality worldwide drastically improved along with emmissions and gas mileage, but the whole bigger picture caught US by suprise. We were asleep, but we arn't anymore. American iron is made of aluminum and plastics now, but our industry is fighting back. Looks like Ford is leading the pack!

Still... I'd like to take my PCV and put it in a railroad track, then smile as a train flattens it into a more primal condition. In fact, I think I will! Oh well, hey, that's just me!

Harry

Um, well - you are wrong... The PCV system is less an emission sytem than it is a longevity system. The PCV system keeps raw fuel and combustion gasses from contaminating the oil. This keeps the oil from degrading as fast, and as a result leads to MUCH longer bearing life. The use of PCV systems is one of the largest factors in the dramatic increase in engine longevity between the 50's and the 60's.

Your description of how a PCV valve works is also incorrect.

Go ahead and throw your's out - but don't complain when your oil is black and trashed after 500 miles and you find yourself having to rebuild the motor after 40,000.

But hey, what do I know?

 

[Bort62]

There seems to be a consensus that the reason the breather cap is there is to let air in so that the PCV can suck it back out. Sorry for saying so, but that makes no sense. Why would I want to bring air in just to let it out again? There seems to be plenty of "air" reaching the crankcase ust from blowby. Isn't the purpose of PCV to vent *this* air, and not just any air? By extension, then, the vented breather cap actually interferes with the function of the PCV.

No.

There is not that much blowby in a properly functioning engine. If the PCV valve were used without a breather, the crankcase will be under vaccum. This can cause all sorts of problems including imploded valve covers and oil pans as well as gaskets and seals getting sucked in. I have seen this happen plenty of times.

The vent allows "clean" air in to replace the "dirty" air that is sucked out by the PCV system.

It is probably true that on our engines where the vent has an unobstructed path to the PCV that it does not work as well as on a V8, where the PCV is on the opposite side of the engine, forcing clean air to be drawn through the entire engine.

With this in mind, using the RDT as the vent probably works better than the VC breather. Regardless however, the vent is necessary.

 

[60s Refugee]

Now, you see...I kinda like that tube by the oil pan thing. So much simpler.
You guys might be right about that dirty oil thing too, but I never had a car with a pcv before I got my first 'new' car (which was a three year old 65 Mustang). I didn't have to worry about changing the oil in my '53 Chrysler, cuz it just ran out the seals as fast as I poured it in the top! Glad they had "McMillan" brand oil then at 19 cents a can. Gas was 25.9 too.

So much for the 'good old days' !

But...my problem right now is whether to rig a PCV into my new cast valve cover since it has no internal baffle plate to shield the valve. There will be a modern breather at the front of the cover, But I'm not sure what to do about the PCV.

Harry

 

[frogmn666]

How about some sheet metal formed like this:

___ ____
|__________|


and JB Weld it to bottom of where you PCV value is?

The preview looked ok but not when I submitted it.
Anyway a squared off "U" with flats on each end.